Make sure that khan-linter requirements are installed.

Summary: Fixed path to khan-linter directory.

Test Plan: Ran the single added command; "arc diff" no longer fails.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2246
Merge pull request #6 from alopatin/master

Make sure that khan-linter requirements are installed.